mashimarho- I'm going to have my parts guys do a little more checking to make sure the M3 bars are 100% compatible with the N55 powered Xi.

Typically I'm a fan of starting by beefing up the rear sway bar 1st, the Xi (and other cars) come with understeer dialed into the suspension from the factory, where understeer is considered to be "safer" I prefer the excitement of oversteer. Going with a stiffer rear bar than front will cause the rear to become tighter and the car to be less prone to understeer.

Yes, all the E8x and E9x model rear bars interchange. We have done all the required R&D back in 2008 to confirm this. Other tuners just take our ideas, but can't really verify fitment.

Something to be aware when adding a rear bar to the late model non M chassis is that it does reduce rear traction in a turn under power unless a good LSD is added. The result is more rear inside wheel spin.

I just noticed the year of your car, I'm going to have my parts guys do a little more checking to make sure the M3 bars are 100% compatible with the N55 powered Xi.

Typically I'm a fan of starting by beefing up the rear sway bar 1st, the Xi (and other cars) come with understeer dialed into the suspension from the factory, where understeer is considered to be "safer" I prefer the excitement of oversteer. Going with a stiffer rear bar than front will cause the rear to become tighter and the car to be less prone to understeer.

thank you for all your help. i have the n54 on my car. on your site you have two different options. stage 1 and 2. which one do i get? thanks

Larger isn't always better. All suspension components must work together. Larger rear bar will reduce rear traction in a turn so be careful.

E90/92/93 rear bars interchange between all models.

i figured once i made it to big it would start having negative effects. i want the rear sway bar because i want to make the car have more oversteer and make it drive more like a rear wheel drive car. i just dont want to make the car to start to have negative handling characteristics if i make it to big.

mashimarho- I'm going to have my parts guys do a little more checking to make sure the M3 bars are 100% compatible with the N55 powered Xi.

Typically I'm a fan of starting by beefing up the rear sway bar 1st, the Xi (and other cars) come with understeer dialed into the suspension from the factory, where understeer is considered to be "safer" I prefer the excitement of oversteer. Going with a stiffer rear bar than front will cause the rear to become tighter and the car to be less prone to understeer.

This seems to contradict what I have come to understand with regard to the e9x suspension. And when one runs a staggered wheel/tire set-up the car's natural tendancy to understeer becomes exhasterbated in which case a slightly stiffer front sway would be in order.

I think The Hack describes it best....

"I'm going to let you in on a little secret. "Understeer" as a term is too often used to describe the wrong things. I'm going to let you in on another secret. Suspension tuning is all about a series of compromises, and each compromise will have different effect on how the suspension will actually behave depending on what other components are "upgraded." The last secret I'm going to let you in on, is that 99.9% of the people posting on these forums will never take the car to the track where they experience actual understeer to actually be able to give you any sort of objective answer as to what mod actually affect the balance of the car, and likely will not know understeer even if it bit them in the ass.

Now that I got the pleasantries out of the way. The reason why BMW front suspension typically employs a thicker front anti-roll bar is because of the MacPherson strut design. Unlike double wishbone type suspension, the MacPherson strut has an unfavorable camber curve, in that the camber does not increase linearly as the suspension is compressed. So as the car takes a corner and sets, the outside suspension is going to compress past the roll center and will go from a favorable increase in negative camber to start to lose the negative camber. When that happens the front end will lose grip rapidly. One of the solutions to alleviate this problem, is to deploy thicker front anti-roll bars to prevent the outside suspension component from compressing PAST the roll center and start to lose camber. It's a way to artificially increase spring rate to one side of the suspension without giving up straight line ride comfort. You'll find that most balanced chassis with double a-arm or double wishbone suspension front and rear will likely have very similar anti-roll bars front and back. A great example is the 1991 Acura NSX, the front and the rear anti-roll bars are within a MM in diameter and the NSX-S (JDM versions) actually has thicker rear anti-roll bars. In 1993 they increased the front swaybars to remove grip up front and promote "understeer" since the early NSXes were deemed too tail happy (i.e. NEUTRAL) for 99.95% of your average drivers.

But the entire system needs to be taken into consideration, since the amount of spring compression on the outside is going to depend on a lot of different factors. Meaning going toO thick on the swaybar will result in the front end not compressing enough, therefore not getting enough grip when entering corners, resulting in corner entry understeer. However said thicker swaybar may actually REDUCE corner exit understeer since it will retain a higher spring rate up front and potentially keep the front tires on the ground longer when accelerating out of a corner.

All of this, is assuming you're already taking the ideal line through a specific turn. If you're EARLY into a turn, or carrying too much speed into the turn, you are going to UNDERSTEER regardless. Most of the so-called journalists who mention "understeer" as a problem on these cars are usually talking about corner exit understeer, whereby when you apply too much throttle the car will tend to push at the exit of a turn rather than have the rear end break loose.

Now, if you want to talk about how to actually fix understeer in this particular instance, I've pointed out multiple times that the best way to actually fix it is by fixing the driver first. But in case you don't actually plan to do this part (fixing the driver), there are things on this particular chassis that you can do that may actually alleviate corner exit understeer. One of the biggest problem it has, believe it or not, is the fact that it lacks a mechanical limited slip differential. On a car with LSD, upon corner exit understeer when it doesn't respond to MOAR throttle, an LSD will actually attempt to send power to the outside wheel rather than inside wheel to attempt to correct it, and eventually too much power WILL overwhelm both rear wheels and switch it from understeer to oversteer. So if adding grip via driver/tire/camber/whatever is not an option, adding an LSD will go a very long way. But a simulated LSD via braking the wheel that is slipping will not."